The present invention relates generally to positioning systems in which an object or user at an unknown location receives signals from a plurality of sources and uses information derived therefrom to determine the object""s or user""s current position. More particularly, the present invention relates to a positioning system which utilises a network of self-integrating positioning-unit devices, synchronised to a Global Navigation Satellite System (GNSS), for high accuracy position determination in satellite obscured environments.
The need to locate exactly where someone or something is on the world""s surface has constantly preoccupied humans. In fact, the precision and predictability with which location can be derived is a yardstick by which a civilization""s technological refinement can be judged. Over time, man has improved terrestrial location and navigation, progressing through sextant and chronometer, inertial systems, LORAN, TRANSIT and, most recently, GPS.
The GPS constellation of 24 satellites created by the United States Government broadcasts precise timing signals locked to on-board atomic clocks. Using precise, well-developed formulae, a user receiver that picks up signals from 3 or more satellites simultaneously can determine its position in absolute global co-ordinates, namely latitude and longitude. GPS has proven to be a boon to location determination because it is globally available, it is reasonably precise, and it is free to the end user.
Despite its technological sophistication, GPS still suffers from several critical limitations that impede its wide adoption at consumer level. Firstly, GPS signal strengths require satellites to be xe2x80x9cin viewxe2x80x9d relative to the receiver. This means that no substantial obstruction can exist between the satellites and the receiver. Secondly, GPS formulae require at least 3 satellites to be in view for determination of a 2-dimensional location (i.e., latitude and longitude), and at least 4 satellites to be in view for determination of a 3-dimensional location (i.e., latitude, longitude and altitude). In combination, these two major shortcomings severely disrupt GPS reliability in built-up areas such as xe2x80x9curban canyonsxe2x80x9d, and they ensure that standard GPS will not function at all inside buildings or in shielded environments. GPS is therefore of extremely limited use in metropolitan environments where a large part of the world""s population lives.
Surprisingly, further xe2x80x9cconsumerxe2x80x9d limitations of GPS arise from its global availability and its potential for reasonably high precision. In its innate form GPS has the potential to deliver an accuracy of approximately 15 meters. The United States Government became concerned with the possibility that their own satellite system could be used against the United States for accurate delivery of enemy weapons payloads. For this reason, signals broadcast by the GPS network for civilian use are intentionally degraded relative to the more accurate, encrypted U.S. military signals. This degradation, commonly called Selective Availability (SA), reduces the raw accuracy available to civilians to approximately 100 meters 2 dRMS.
In an effort to overcome SA, a system known as Differential GPS (DGPS) was developed for civilian users in a localized area. DGPS is capable of giving accuracy of several meters to a mobile user. However, DGPS demands the establishment of an expensive local broadcasting station. It also necessitates the mobile consumer to purchase additional equipment, in the form of a radio receiver, to acquire DGPS corrections for their GPS receiver. A further recent development called Real Time Kinematic (RTK) allows accuracy from the GPS system to be improved to approximately one centimeter. Whilst this degree of accuracy is highly desirable for many possible applications, RTK is almost wholly the province of highly technical and skilled disciplines such as geodesy, surveying, and physics. RTK receivers are commonly an order of magnitude more expensive than standard GPS receivers are. RTK systems require uncommon local transmitters, and, depending upon the level of complexity can take up to 10 hours of motionless signal acquisition before RTK-accurate positions can be determined. The level of expense necessary for RTK, along with the specialized equipment and skills required, strongly militate against RTK being considered for consumer or commercial use.
In summary: GPS is a marvelous boon to modem location and navigation needs. However, GPS is optimally employed in open field, desert or high-seas environments. Its usefulness is severely compromised in urban canyons, and it was never designed to work indoors. Moreover, should sufficient GPS signals ultimately be acquired in built-up areas, the resultant position solution is so highly degraded by SA that it may prove of little use in restricted areas. If a consumer in this situation, looks to improved accuracy via DGPS or RTK methods, it is only achievable with considerable effort, expense and relatively complex infrastructure.
Attempts to overcome these difficulties, are described in prior art. Hybrid systems have been developed which incorporate an absolute positioning system (e.g., GPS) plus a relative positioning system. Such methods include inertial sensor systems that incorporate xe2x80x9cdead reckoningxe2x80x9d when satellites are obscured (U.S. Pat. No. 5,311,195) or commercial radio broadcast transmissions performing xe2x80x9cdelta phase positioningxe2x80x9d when satellites are obscured (U.S. Pat. No. 5,774,829). 
Unfortunately, these prior art systems have several drawbacks. Dead reckoning exhibits cumulative error with extended use and both dead reckoning and delta phase position accuracy is limited to initial absolute position accuracy. Any initial position ambiguity will therefore be carried on through the subsequent position solutions. Delta phase position accuracy will be constrained by pre-existing geometry of commercial radio broadcast transmission sites. Poor geometry as seen by the roving receiver will produce poor position solutions. In addition, delta phase position accuracy is constrained by the frequency/wavelength of the transmission signal whereby lower frequencies (i.e., longer wavelengths) produce decreased accuracy. Moreover, delta phase roving receivers need a pre-existing knowledge of commercial radio broadcast transmission site co-ordinates. Finally, delta positioning requires a reference receiver and data link in addition to the commercial radio broadcast transmissions. U.S. Pat. No. 5,774,829 suggests that this data link be placed as information on the commercial radio broadcast transmission signal SCA channel. This would potentially require co-operation with thousands of commercial broadcasters, bringing about substantial logistics problems.
Also known in the art are attempts to use pseudo-satellites, or xe2x80x9cpseudolitesxe2x80x9d, to enhance or augment the standard GPS constellation. Pseudolites are ground based transmitters that emit GPS-like signals. Pseudolites were first used in 1977 by the US Department of Defense for Phase I GPS testing at the Yuma Proving Ground in Arizona. They were used to augment the GPS constellation for testing user equipment before there were sufficient satellites for navigation. In 1984 Klein and Parkinson were the first to point out that pseudoiites could be a useful adjunct to the operational GPS system, improving navigation availability and geometry for critical applications such as aviation. In 1986 Parkinson and Fitzgibbon developed and demonstrated a procedure for finding the optimal location for a ranging pseudolite. Also in 1986 the RTCM-104 committee, which developed the first standard for local area DGPS systems, proposed a method for transmitting DGPS information by pseudolite.
Pseudolites are currently expensive devices and are manufactured in extremely small quantities. They generally transmit their signals on the GPS L1 and L2 frequencies, so they normally need regulatory approval to operate. Experimental groups within Universities, government agencies, the military or very large companies have therefore customarily used pseudolites. So whilst these devices have been known for a long period of time, their use in general location and navigation is extremely rare. The prior art reflects the limited availability of pseudolites.
Several industries have used pseudolites to enhance GPS signals in localised areas. Aviation use of pseudolite devices is typified by U.S. Pat. No. 5,572,218 which describes a method of placing a pseudolite at the end of a runway below the final approach path of an aircraft. This successfully allows extremely fast integer cycle ambiguity resolution, generating very precise positioning. U.S. Pat. No. 5,375,059 is representative of how companies like Caterpillar have applied pseudolites to open-pit mining, which is one of the more typical applications of these devices. These systems employ conventional local-area pseudolite/reference station configurations well known in the art.
U.S. Pat. No. 5,686,924 xe2x80x9cLocal-area position navigation system with fixed pseudolite reference transmittersxe2x80x9d and U.S. Pat. No. 5,708,440 xe2x80x9cPseudolite translator for unlicensed frequenciesxe2x80x9d(both to Trimble, et al.) jointly describe augmentation of GPS signals over a local area. This local area system has no clear provision for pseudolite/reference station integration and therefore lacks the fundamental prerequisite of time coherence for accurate position determination.
One prior art reference is known that specifically generates GPS signals indoors. U.S. Pat. No. 5,815,114 (Speasl, et al.) describes a pseudolite system positioned in an entirely shielded environment. This system uses signals regenerated by a computer processing unit. These signals are distributed via coaxial cable to four pseudolites in an area within a building. This local area system requires an extremely complex and extensive installation, as well as total shielding from the GNSS constellation to ensure original and regenerated signals do not conflict.
All of these prior art citations disclose rudimentary pseudolite systems used in local-area, closed systems.
No prior art discloses a method or device, nor teaches techniques, which: (a) allow seamless integration of a network of terrestrially-based positioning-unit devices into a GNSS system, and; (b) allow substantially endless propagation of positioning-unit devices over a substantially unlimited area.
The necessity for extremely precise location services in built-up areas is growing rapidly. The proliferation of hand-held consumer devices and the desire for location-dependant information are disclosing a need for a workable, integrated and complete solution. In hand-held applications, the SA degraded accuracy of standard GPS is unsatisfactory; meter level accuracy or better is essential. A system that would allow seamless transition from outdoors to indoor, without necessitating different location technologies, is highly desirable. A system that would propagate itself, and then allow continued expansion, both indoor and outdoor, is also highly desirable. Further, integration with a world standard system like GPS would bring synergies to the system in the form of readily available and standardized components, simplified manufacturing and use of well-known techniques. A system that provides these benefits to the public at consumer-level prices, without expert assistance for infrastructure construction, is also most desirable. The prior art does not meet all these manifest needs.
Hence, it is a goal of the present invention to overcome the above stated disadvantages of GPS positioning, as well as providing a system that compliments, enhances and extends GPS-style techniques using entirely novel methods.
It is, therefore, an object of the present invention to improve position location systems.
Another object of this invention is to increase the number of situations in which a GNSS-based position location system may be used.
It is a further object of this invention to disclose an open-architecture network of terrestrially-based positioning-unit devices that can be propagated over an unlimited area.
Still another object of this invention is to employ GNSS-like signals throughout a terrestrially-based positioning network.
It is still a further object of this invention to teach a method of creating and propagating a terrestrially-based positioning network.
Yet another object of this invention is to provide a network of positioning-unit devices that can integrate seamlessly with a GNSS constellation.
An additional object of this invention is to provide a network of positioning-unit devices that can synchronise with a GNSS constellation.
Yet a further object of this invention is to provide a positioning network which will allow seamless transition from a GNSS-based location to a network-based location, or derive a location from any apportionment of each location system.
It is yet a further object of this invention to provide a positioning network, which will allow seamless transition from a GNSS-based location to a network-based location, or derive a location from any apportionment of each location system, utilising a unitary roving device attuned to each location system.
Yet another object of this invention is to provide a network of positioning-unit devices that can augment GNSS location systems within urban canyon environments.
Still yet another object of this invention is to provide a network of positioning-unit devices that can extend GNSS-style absolute positioning into satellite obscured areas, inside buildings and other structures, and into other environments where traditional GNSS has not previously functioned.
It is yet still a further object of the present invention to disclose a positioning-unit device that can self-survey and self-integrate into the GNSS constellation and/or current network of positioning-unit devices, thereby providing both absolute and relative positioning in any satellite obscured or indoor environments.
Still another further object of the present invention is to teach protocols for initiating and maintaining communications between positioning-unit devices, thereby allowing network information to be passed between positioning-unit devices.
It is yet another object of the present invention for each positioning-unit device to include a reference receiver to provide distributed differential corrections throughout the network.
It is yet another further object of the present invention to provide Real Time Kinematic positioning throughout the network, thus providing centimeter accuracy.
In additional object of the present invention is to provide pseudorange and carrier phase measurements for both GNSS satellites and Network Positioning System (NPS) positioning-unit devices, thus providing both meter and centimeter position accuracy.
Still another object of the present invention is to provide dual frequency pseudolite transmissions which allow single epoch carrier phase integer ambiguity resolution within environments enhanced by positioning-unit devices.
It is still a further object of the present invention to provide triple frequency receivers to accept GNSS signals and two additional pseudolite signals.
Yet another object of the present invention is for positioning-unit devices to accept WAAS differential corrections so as to provide differential corrections when no network differential corrections are available.
An additional object of the present invention is to provide a positioning-unit device that incorporates current GNSS technology to maintain substantial compatibility between systems.
Still yet another object of this invention is to provide a network of positioning-unit devices that may be propagated by non-technical personnel, thereby eliminating the expensive infrastructure requirements and the need for specialist skills.